Solar System Size Calculator
How big a system you need to cover your actual electricity use — in kW, panels, and roof space.
✏️ These numbers were filled in from a shared link — change anything to make it your own.
How sizing works
system kW = (annual kWh × offset target) ÷ production per kW
panels = system kW × 1,000 ÷ panel wattage (rounded up to a whole panel)
Because you can only buy whole panels, the real system is slightly larger than the exact figure — which is why the "actual offset" above can exceed your target.
Find your real usage
Do not guess from a single month. Pull twelve months of kWh from your utility bills or online account and average them — summer air conditioning and winter heating swing usage dramatically, and sizing off a mild month leaves you short all year.
If you are about to add an EV, a heat pump, or a pool pump, add that load before sizing. It is far cheaper to include panels in the original install than to add them later.
Got a size? Run it through the Solar Payback Calculator to see when it pays for itself.
Estimates only. Roof area assumes about 18 sq ft per panel and does not account for shading, obstructions, orientation, tilt, or setback requirements. Cost per watt varies widely by installer and region — get multiple quotes. General information, not financial advice.
Frequently asked questions
How many solar panels do I need?
It depends on how much electricity you use, how much sun your roof gets, and how powerful each panel is. Divide your annual kWh usage by the annual production per kW in your area to get the system size, then divide by the wattage of the panels you are quoted. A typical US home lands somewhere around 15–25 panels.
Should I size for 100% offset?
Not always. Many utilities pay less for exported electricity than they charge for imported, so overbuilding past your own usage earns a poor return on the excess. Sizing to roughly your actual consumption is usually the sweet spot — unless you are about to add an EV, a heat pump, or air conditioning, in which case size for the load you are about to have.
How much roof space does solar need?
Roughly 18 square feet per panel, so a 20-panel system needs around 360 square feet of usable, unshaded roof. Usable is the key word: chimneys, vents, dormers, setback requirements, and shading all reduce what actually counts, so treat the figure here as a minimum.
What is production per kW and how do I find mine?
It is how many kWh each kW of panels generates per year where you live, and it drives everything. It ranges from roughly 1,100 kWh in the cloudy Northeast to over 1,700 in the desert Southwest. NREL’s free PVWatts tool gives a location-specific estimate — use that rather than a national average if you can.